1. Field of the Invention
Apparatuses and methods consistent with the present general inventive concepts relate to an image forming apparatus. More specifically, the present general inventive concepts relate to an image forming apparatus using a transfer belt.
2. Description of the Related Art
In general, an image forming apparatus is an apparatus, which transfers a developer image formed on an image carrier (for example, a photosensitive drum) onto a printing medium to form a monochrome or color image, such as, for example, a laser printer, a copy machine, a facsimile and so on.
In case of an image forming apparatus for embodying the color image, a transfer belt, which first receives the developer image from the image carrier before it is finally transferred onto the printing medium, is frequently used. The image forming apparatus using the transfer belt may be divided into a single pass type apparatus having a plurality of image carriers and a multi-pass type apparatus having a single image carrier.
The image carrier(s) is disposed on an outer side of the transfer belt, and a primary transfer roller(s) having the same number as that of the image carrier(s), a driving roller to drive the transfer belt, a driven roller to support the transfer belt along with the driving roller and so on are disposed on an inner side of the transfer belt. A secondary transfer roller is disposed opposite to one of the driving roller and the driven roller. Here, the roller, which is disposed opposite to secondary transfer roller, is usually called a backup roller.
The primary transfer roller is disposed opposite to the image carrier with the transfer belt interposed therebetween, or disposed to have an offset from the image carrier, that is, to be spaced apart from the image carrier in a rotating direction of the transfer belt, the former being referred as a direct form and the later being referred as an indirect form.
In case of the indirect form, there are advantages in that because there is no concern about a damage of the image carrier by a press of the primary transfer roller, a metal roller can be used as the primary transfer roller, thereby allowing reduction in cost of the primary transfer roller and preventing slip between the transfer belt and the driving roller by nonmetallic dregs (e.g. sponge dregs), which are generated from a nonmetallic roller, such as a sponge roller, as compared with a case of using the nonmetallic roller.
In the indirect form, the primary transfer roller (a most-downstream primary transfer roller in case of the single pass type), the image carrier (a most-downstream image carrier in case of the single pass type) corresponding to the primary transfer roller, and the backup roller are disposed in a rotating direction of the transfer belt in order. In addition, the primary transfer roller (the most-downstream primary transfer roller) and the backup roller may be configured in a structure disposed adjacent to each other. In this case, not all of transfer current induced in the primary transfer roller (the most-downstream primary transfer roller) by a bias voltage may flow to the image carrier, but rather a portion of the transfer current may flow to the backup roller adjacent to the image carrier.
The portion of the transfer current flowing toward the backup roller from the primary transfer roller as described above is usually called a ‘stray current’. If the stray current is unduly high, defects may occur in image during the primary transfer or secondary transfer.
According to a design of the image forming apparatus to satisfy a demand for size reduction, a distance between the primary transfer roller and the backup roller is also downsized, thereby causing the above stray current to increase.
Like this, the demand for size reduction in the image forming apparatus may generally collide with the problem caused by the increase of the stray current.